Recently, various kinds of optical elements with a very small structure, of which the feature size is as small as the wavelength of visible light (that falls within the range of about 380 nm to about 780 nm) and which is sometimes called a “sub-wavelength structure (SWS)”, have been developed. If an uneven pattern, of which the feature size is equal to or smaller than the wavelength of incoming light, is formed on the surface of a substrate, then reflection of the light from the surface can be reduced. The size of those recesses and projections that form such an uneven pattern may be set to fall within the range of 10 nm to less than 500 nm, for example.
Among other things, an antireflection technology based on the principle of a moth-eye structure has attracted a lot of attention these days. With the moth-eye structure, the refractive index can be changed virtually continuously in those recesses' or projections' depth (or height) direction. That is to say, the surface with the moth-eye structure has no interfaces which have substantive refractive index differences in the light traveling direction, and therefore, can reduce the reflection of light effectively enough.
The moth-eye structure is advantageous because the structure not only performs an antireflection function with small incident angle dependence over a wide wavelength range but also is applicable to a lot of materials and contributes to forming an uneven pattern on the substrate directly. That is why by adopting the moth-eye structure, a high-performance antireflection film can be provided at a lower cost.
The moth-eye structure may be formed using an anodized porous alumina layer (porous oxide film) which is obtained by anodizing an aluminum base (see Patent Document No. 1, for example). Hereinafter, such an anodized porous alumina layer obtained by anodizing an aluminum base will be described briefly.
If an aluminum base is immersed in an acidic electrolytic solution of sulfuric acid, oxalic acid, phosphoric acid, or any other appropriate acid, and if a voltage is applied thereto using the aluminum base as an anode, oxidation and dissolution will advance concurrently on the surface of the base. As a result, an oxide film with a huge number of nanopores (i.e., a porous aluminum layer) is formed over the surface of the base.
Those nanopores will be formed in the shape of circular cylinders perpendicularly to the oxide film. In addition, if the anodization process is carried out under a predetermined condition, those nanopores will get self-organized and arranged in a regular pattern. Consequently, by performing such an anodization process on aluminum, a very small structure with various regular patterns can be formed relatively easily and cost-effectively.
The applicant of the present invention discloses, in Patent Document No. 1, a method of forming an antireflection film (antireflective surface) using a stamper which has an anodized porous alumina film over its surface. Patent Document No. 1 also discloses a technique for forming an antireflection film using an alumina layer, of which the very small recesses have a stepped side surface.
If the porous alumina film is used, a mold for forming a moth-eye structure on the surface (which will be referred to herein as a “moth-eye mold”) can be made easily. In particular, as disclosed in Patent Document 1, if the surface of the anodized aluminum film is used as a mold as it is, the manufacturing cost can be cut down significantly, which is beneficial.
An antireflection film can be made just by pressing the moth-eye mold against a photocurable resin that has been applied onto a transparent film, for example. More specifically, the uneven surface of the moth-eye mold is pressed against the photocurable resin in a vacuum. Then, the photocurable resin is irradiated with an ultraviolet ray with the moth-eye mold still pressed against it. After that, when cured, the resin is released from the moth-eye mold. Such a method of transferring the surface shape of a mold onto a resin while curing the resin with an ultraviolet ray is called a “UV imprinting technique”.